An electrodeless, low pressure (e.g. fluorescent) lamp incorporates a hermetically sealed vitreous envelope that typically contains a metal vapor and a rare gas. The envelope has an external chamber into which an excitation coil is received. The excitation coil electrically excites the metal vapor in the vitreous envelope to emit light by passing a high frequency electromagnetic field through the vitreous envelope; without any electrodes within the envelope itself, the lamp is electrodeless. The high frequency electromagnetic field, however, can create undesirable electromagnetic interference (EMI) on the mains, or power lines, that supply electric power to the lamp.
To reduce such EMI to an acceptable level, the prior art has taught the use of a transparent, conductive coating on the interior of the envelope for suppressing the EMI. A cooperating, conductive coating on the exterior surface of the vitreous envelope is used to capacitively couple the inner conductive coating to a circuit potential that is suitable for suppressing EMI on the power mains. Being located on the exterior surface of the vitreous envelope, however, the outer conductive coating is susceptible to damage through any of physical abrasion, reaction with chemicals in the vitreous envelope, or reaction with chemicals in the external environment (e.g. H.sub.2 O present in humidity). For instance, vacuum-deposited aluminum has been found by the present inventors to adversely react with a vitreous envelope formed of soda-lime-silicate glass, losing its structural integrity. The soda-lime-silicate glass is desirable for the vitreous envelope due to its low cost. Additionally, the outer conductive coating must be temperature tolerant, and be able to withstand thermal shocks and thermal cycling, such as occur during lamp operation.
Briefly, a suitable, EMI-suppressing, outer conductive coating should, over the expected life of a lamp, retain adequate electrical conductivity and provide appropriate capacitive coupling with the inner, transparent, conductive coating mentioned above. To assess such durability of the conductive coating, the present inventors have subjected a large number of coatings to durability tests measuring the following factors: (1) resistance to abrasion; (2) resistance to a humid environment; (3) temperature tolerance (e.g. 160.degree. C.); and (4) resistance to thermal shock and thermal cycling. Most coatings that were tested failed to meet the foregoing durability criteria. The present invention is directed to coatings that meet the durability criteria.